Nowadays, coated steel sheets in which corrosion resistance is given to the base steel sheet, in particular, galvanized steel sheets or galvannealed steel sheets which are excellent in terms of corrosion resistance, are used in the fields of, for example, automobile, domestic electric appliance, and building material.
Generally, a galvanized steel sheet is manufactured using the following method. First, a steel sheet produced by hot-rolling and cold-rolling a steel slab, or followed by heat treatment, is annealed for recrystallization in a non-oxidizing atmosphere or a reducing atmosphere after cleaning the surface of the steel sheet using a degreasing method and/or a pickling method in a pretreatment process, or removing oil on the surface of the steel sheet by combustion in a preheating furnace without performing the pretreatment process. Then, the steel sheet is cooled to a temperature suitable for galvanizing in the non-oxidizing atmosphere or the reducing atmosphere and dipped into a galvanizing bath into which a small amount (about 0.1 to 0.2 mass %) of Al is added, without being exposed to air. With this method, the surface of the steel sheet is galvanized so that a galvanized steel sheet is obtained. In addition, a galvannealed steel sheet is obtained by performing a heat treatment on the galvanized steel sheet in an alloying furnace.
Nowadays, in the field of automobile, since a decrease in weight of steel sheet is promoted along with an increase in performance of steel sheet, a high-strength galvanized steel sheet having corrosion resistance is increasingly being used. An increase in strength of steel sheet is realized by adding chemical elements for solid solution hardening such as Si and Mn. In particular, since Si has the advantage of increasing the strength of steel without decreasing its ductility, a Si-containing steel sheet has potential as a high-strength steel sheet. On the other hand, in the case where a galvanized steel sheet or a galvannealed steel sheet is manufactured using a high-strength steel sheet containing a large amount of Si therein as a base steel sheet, the following problems exist.
A base steel sheet is subjected to annealing in a reducing atmosphere before galvanizing as described above. However, since Si in steel has a high affinity for oxygen, Si is selectively oxidized, even in a reducing atmosphere, so as to form oxides on the surface of the base steel sheet. Such oxides decrease the wettability of the base steel sheet with molten zinc, which results in coating defects at galvanizing. In addition, even in the case where coating defects do not occur, there is a problem of decrease in coating adhesiveness.
Moreover, such oxides significantly decrease an alloying rate in an alloying process after galvanizing. As a result, there is a significant decrease in productivity of galvannealed steel sheet. On the other hand, in the case where alloying treatment is performed at a high temperature in order to achieve high productivity, since there is a problem of decrease in powdering resistance, it is difficult to achieve efficient productivity and sufficient powdering resistance at the same time. In addition, since there is a decrease in stability of residual γ phase by performing an alloying treatment at a high temperature, there is a decrease in effect of adding Si. As described above, it is very difficult to manufacture a high-strength galvanized steel sheet having satisfactory mechanical properties and coating quality at the same time.
In order to solve such problems, some techniques are disclosed. Patent Literature 1 discloses a technique in which the wettability of the base steel sheet with molten zinc is increased as a result of forming a reduced iron layer on the surface of the base steel sheet by performing a reduction annealing after forming oxidized irons on the surface of the base steel sheet in an oxidizing atmosphere. In addition, Patent Literature 2 discloses a technique in which satisfactory coating quality is achieved by controlling oxygen concentration in an atmosphere at preheating. Moreover, Patent Literature 3 discloses a technique for manufacturing a galvanized steel sheet having good surface appearance without coating defects or pressing flaws by dividing a heating zone into three zones called A to C zones and appropriately controlling the temperatures and the oxygen concentrations respectively of the three zones.